The manufacture of electronic components such as semiconductors, storage devices, and flat panel displays often takes place within a series of individual processing stations that perform specific operations such as deposition, etching, thermal processing, and cleaning. Stand-alone tools include a single processing station that can be connected to a loading station through a platform. Cluster tools include two or more processing stations that are connected with each other and to a loading station through a similar platform. Robotic wafer handlers mounted on the platforms move wafers, comprising various substrates, between the loading and processing stations.
In most such tools, the platforms are enclosed by a vacuum chamber; and valve gates control access between the vacuum chamber and the individual loading and processing stations. When closed, the valve gates isolate the stations for processing the wafers in discrete environments. The stations are evacuated prior to opening their respective valve gates to prevent the spread of contamination into the platform. When opened, the valve gates permit the exchange of wafers in various stages of processing between the stations within a partial vacuum.
The loading stations, which are also referred to as "load-lock chambers", can be sized to hold different numbers of wafers from a single wafer to one or more cassettes containing up to 25 Wafers each. Also, more than one loading station can be used for holding wafers in each tool; and one such station can be used for holding unprocessed wafers, and another such station can be used for holding processed wafers.
The robotic handlers generally have an articulated arm with an end effector for carrying a wafer. Drives provide for rotating the arm around a vertical axis, translating the arm short distances along the vertical axis, and translating the end effector portion of the arm along a horizontal axis. Generally, the handlers transport one wafer at a time. For example, the handlers pick up one wafer from a processing station, return it to a loading station, pick up another wafer from the loading station, and transport it to the processing station.
Such handling between operations delays value-added processing of wafers within the stations (i.e., the actual modification of wafers) and adds to the total time required to process wafers within the tools. In fact, the total processing time often exceeds the value-added processing time by a factor of four or more. This depresses production rates and adds to the cost of finished products.
Robotic handlers have also been made with two opposing arms having respective end effectors separated by 180 degrees. During processing by one of the stations, the end effector of one of the arms picks up a wafer from a loading station or a prior processing station. When processing is completed, the empty end effector extends into the processing station, picks up the processed wafer, and retracts. The two arms are then rotated through 180 degrees, and the other end effector is extended for placing its wafer into the same station.
Although such two-arm handlers are faster than one-arm robots for reloading wafers in processing stations, considerable time is wasted by requiring rotations through 180 degrees and by the separate extension of both end effectors into the processing stations. The two-arm handlers are also more costly, requiring more complex controls for positioning the two effectors.